Previously, configuration files for Zabbix agent 2 plugins used to support the parameter:
Plugins.<plugin name>.Capacity=<int value>
 
Now it is deprecated and removed from the plugins created by Zabbix developers. If a user tries to configure it the plugin *.conf file for a plugin provided by Zabbix, an error happens on Zabbix agent 2 start:

zabbix_agent2 [481161]: ERROR: cannot register plugins: [PostgreSQL] Cannot assign configuration: invalid parameter  at line 10: unknown parameter 

But it is still possible to configure this parameter in user-created plugins if it is defined in plugin\config.go PluginOptions struct in the user-created plugin.
 
A new parameter is recommended instead. The Zabbix agent 2 plugin scheduler in the main Zabbix repo in "src/go/internal/agent/scheduler/manager.go" treats it as a higher priority one:
Plugins.<plugin name>.System.Capacity=<int value>
 
The old parameter should be completely removed from the Zabbix main repo in a new Zabbix release. This is a breaking change and it should not be backported.